Applying a fluoropolymer film to a body

ABSTRACT

A thin fluoropolymer film is covalently bonded to a microporous ptfe film to make a bilayer for separation, filtration or reverse osmosis, by exposing the microporous film to perfluorocyclohexane under plasma.

This invention relates to applying a fluoropolymer film to a body,especially to porous and microporous films of fluoropolymers, andextends to coated bodies and two layer films

Microporous films and membranes from polymers are well known, andasymmetric forms find wide application in filtration and separation.Their manufacture is typically undertaken by a variety of castingprocesses and other relatively straightforward techniques allowable bythe physical and chemical nature of the polymer. Polymers amenable tosuch straightforward techniques however are thermally, chemically andsometimes physically inferior to the more stable fluoropolymers, e.g.polytetrafluoroethylene (ptfe). Fluoropolymers are selected for theirinertness and chemical resistance, and these very properties make itdifficult to bond layers of fluoropolymers together. The techniques usedfor processing ptfe owe more to powder metallurgy than plastics as thematerial is not a true thermoplastic. The manufacture of such componentsmost usually involves a compression moulding stage and a heat treatmentor sintering stage.

PCT Publication NO 90/13593 discloses a mechanical bonding method forporous ptfe layers which are impregnated with perfluoro ion exchangepolymer, and further refers to numerous earlier patents in the field.Such a mechanical bond may not be adequate for all applications.Japanese Laid-Open (Kokai) 62-204826 discloses coating a porous ptfemembrance in a plasma vessel by introducing gaseoustetrakis(trifluoromethyl)dithioethane, which forms a polymer in the formof a thin film on the membrane. This introduces sulphur into the productas well as --CF₃ groups at the surface, which is unnecessarilyhydrophobic for some applications.

These publications do not teach any way of making a well bonded bilayerof pure fluoropolymer. Such a bilayer could find application infiltration, separation or reverse osmosis.

It would thus be desirable to solve the problem of formation of a verythin continuous layer of fluoropolymer strongly bonded to the surface ofa microporous fluoropolymer substrate.

According to the present invention, a method of applying a fluoropolymerfilm to a body comprises exposing the body to fragments exclusively ofthe formula --C_(n) F_(2n) --, under conditions whereby the fragmentscombine on the surface of the body to form an adherent fluoropolymerlayer. Also according to the invention, a method of applying afluoropolymer film to a body comprises exposing the body to a supply ofsaturated molecules of the formula C_(n) F_(2n), causing scission of themolecules, and allowing the fragments to combine on the surface of thebody to form an adherent fluoropolymer layer. The body may becarbonaceous polymer e.g. a fluoropolymer such as ptfe, optionallyitself a film, which may be porous or microporous, in which case thelayer will be covalently bonded thereto. The molecules may becyclo-perfluoroalkanes e.g. C_(n) F_(2n) where n=4-8, preferably 6. Thereason for preferring perfluorocycloalkane is that It can undergoscission affording only multiple CF₂ units, in particular no CF₃fragments at all. This cannot occur with non-cyclic saturatedfluoroalkanes. This allows the product to be as close to ptfe, i.e. CF₂linkages, as possible, avoiding multiple CF₃ fragments, which are morehydrophobic than CF₂. Of the perfluorocycloalkanes, the butane tends toinstability, the pentane is possible, while the heptane and octane arebecoming exotic for no real gain. The hexane is therefore the mostpreferred, from cost, stability, availability and volatility points ofview.

The body may be etched with a noble gas plasma e.g. argon at say 10-30W,for the purpose of cleaning, before the film is applied. Thereafter, thebody may be subjected to a somewhat gentle plasma irradiation,preferably <5W, e.g. 0.1-50W, in a chamber which may be evacuated to0.01 to 5 torr, such as 0.2 to 0.3 torr, of fluorocarbon. Expressed interms of unit area-to-be-coated of the body, preferred plasma powers are<100W/m², e.g. 2-1000W/m². Lower powers lessen undesirablecross-linking.

As a new product in its own right, the invention provides a carbonaceouspolymer (e.g. ptfe) body covalently bonded to a fluoropolymer film.Likewise as a new product, the invention provides a two layerfluoropolymer film containing no atoms other than of carbon and halogenand which cannot be delaminated by hand.

A specific embodiment of the invention will now be described by way ofexample, for producing a continuous film of plasma polymer upon amicroporous polymeric substrate.

Microporous ptfe film manufactured by the Mupor™ procedure, EuropeanPatent 247771, is the substrate to be coated. A 0.06m² sample of it isplaced in an enclosure which is then evacuated to low pressures, about0.05 torr, to remove air and moisture. Plasma of power 5W is thengenerated in the enclosure via say high voltage, 3000 to 40000 volts orby a high frequency generator, say 10MHz or 13.56MHz.Perfluorocyclohexane at 0.2 torr is introduced into the cavity at 0.2ml/mln.

Under these conditions very reactive species are produced which in turnreact with the surface of the article, which reactive sites can then inturn react with monomeric species introduced into the enclosure. Theexperimental conditions required will vary from one system to anotherand the techniques and durations employed similarly can be varied tosuit individual requirements, e.g. `etching` where surfaces can becleaned by the gradual erosion of the surface by reactive species,plasma polymerisation and plasma initiated `grafting`. The cyclo-C₆ F₁₂is subjected to a sufficiently high electron voltage to generateperfluoro fragments, e.g. CF₂, C₂ F₄, C₃ F₆. These species then react toform a layer of plasma polymer across, and covalently bonded to, thesurface and, in so doing, fill the pore entrances, eventually buildinginto a controlled continuous thin film, the process being terminatedwhen the required properties have been attained. Coating a ptfe filmhaving 4-micron pores under these conditions for 10 minutes yielded acoating several microns thick completely sealing the pores. For typicalsmaller-pore films, a useful product may be attained in say 2 minutes.

It will be noted that the process occurs in the gas phase under verymild conditions. The plasma generated within the cyclohexane atmospherecreates active fragments (radicals etc) based on CF₂ units whichpolymerise and attach to the surface of the membrane in situ, whichitself remains at a temperature of around 300K.

Clearly careful control will ensure the thinnest continuous layer tomaximise the aqueous flow rates during e.g. reverse osmosis separationsof saline or brackish water.

Such materials have great utility in the field of filtration andseparation allowing for the first time a membrane filter with thechemical, biological and thermal advantages of ptfe but withadvantageous flux rates associated with the very thin active layer.

Other so-called anisotropic ptfe filters have poor bond strength betweenthe substrate and the active layer. This technique not only allows greatcontrol over the film properties but ensures the strongest possibleadhesion to the substrate.

The process is both rapid and cost effective and additionally has wideapplicability in the separation field. For example composite materialscan be manufactured with great savings, e.g. in those situations wherethe active layers are very expensive a lower cost substrate can be usedthus minimising the quantity of the active layer.

A further application embodies dissimilar monomeric species attached toboth sides of the substrate, and additionally the technique is equallyeffective on other geometries, e.g. tubular and granular forms of thesubstrate ptfe. This now allows separations of materials in areas ofchromatography normally restricted to silica-based phases.

I claim:
 1. A method of applying a fluoropolymer film to a porous ormicroporous film, comprising exposing the porous or microporous film toplasma irradiation wherein the plasma power is less than 5 w and to asupply of saturated molecules of the formula C_(n) F_(2n), where n isfrom 4 to 8, causing scission of the molecules, whereby fragmentscombine on the surface of the porous or microporous film to form anadherent fluoropolymer layer.
 2. A method of applying a fluoropolymerfilm to a porous or microporous film of carbonaceous polymer, comprisingexposing the porous or microporous film to plasma irradiation whereinthe plasma power is less than 5 w and to a supply of saturated moleculesof the formula C_(n) F_(2n), where n is from 4 to 8, causing scission ofthe molecules, whereby fragments combine on the surface of the porous ormicroporous film to form a fluoropolymer layer covalently bonded to thefilm.
 3. A method according of claims 1 or 2, wherein the molecules areof cycloperfluoroalkane.
 4. A method according to claims 1 or 2, whereinthe molecules are of cyclo-C_(n) F_(2n) where n=4, 5, 7 or
 8. 5. Amethod according to claims 1 or 2, wherein the molecules are ofcycloperfluorohexane.
 6. A method according to claims 1 or 2, whereinthe porous or microporous film is etched before the application of saidfluoropolymer film thereto.
 7. A method according to claims 1 or 2,performed in a chamber evacuated to 0.01 to 5 torr.
 8. A methodaccording to claims 1 or 2, wherein the porous or microporous film ispolytetrafluoroethylene.